Two British besties on a nerd road trip pick up a fugitive alien of the green-skin variety in Paul, an absolutely A-dorable, cute-as-a-button send-up of sci-fi-geek culture.

Graeme Willy (Simon Pegg) and Clive Gollings (Nick Frost) love all things sci-fi. Clive even won an award for his own sci-fi story a few years back, but now hasn’t quite finished his book, the main drawing power of which seems to be the three-boobed alien woman on the front. But they are having a delightful time meeting more successful sci-fi authors, buying assorted memorabilia and hanging out with the assembled Wookies and Princess Leahs at the Comic-Con in San Diego. And, after the movies and the video games, there’s still more fun to be had. The pair, on a long-planned vacation, rent an RV and head out on a road trip of The-Truth-Is-Out-There-ish hot spots: Area 51, Roswell, etc.

The trip isn’t all tea and crumpets — Graeme and Clive run into a fair amount of dust-covered yokels who aren’t sure if the pair are uber-nerds or a couple but want to beat them up either way. And then there’s the night that a car crashes in front of them, tumbling off the road. As they stop to see if anyone’s been hurt, they see the car’s sole occupant, Paul, uninjured and lighting a cigarette. To his shame for the rest of the movie, Clive faints and wets himself because Paul (voice of Seth Rogen) is a big-headed, big-eyed, green-skinned alien.

Paul explains that he learned that the government is preparing to end his stay on Earth with a genuine alien autopsy and decided to escape from his classified hideout. Graeme and Clive agree to take him to an unspecified destination, where he hopes to get a ride home. But, of course, the government isn’t so keen on this plan and soon Man in Black-type Agent Zoil (Jason Bateman) and witless FBI agents Haggard (Bill Hader) and O’Reilly (Joe Lo Truglio) are on the trail of Paul and company. And that’s not all — as Graeme and Clive are forced to make mad dashes through the cowboy-ish West, they inadvertently make other enemies, including Moses Buggs (John Carroll Lynch), an RV park operator and the fundamentalist Christian father of Ruth Buggs (Kristen Wiig), a woman Graeme and Clive accidentally kidnap.

Cross this setup with the sensibilities that made Hall Pass, to pick on a recent example of boneheaded dude humor, and you have something unwatchable — bigness and meanness all over the place. But Paul is — and maybe this isn’t exactly what its makers would want to hear — the sweetest, most charming comedy I’ve seen in a while. It loves its nerd characters with its whole, Spock-ears-wearing heart. It loves aliens and sci-fi and the goofiness of the genre. It pokes fun at E.T. and Star Wars (a character shooting his police-radio and saying “Boring conversation anyway” might actually be my favorite thing in movies so far in 2011) in a way that also celebrates these things and their fans. It is never cruel, never homophobic or misogynist as buddy comedies like this can sometimes be.

At first this much cuteness felt like it was going to turn the movie to mush, that we’d have all softness, no humor. But I laughed out loud plenty, found myself enjoying the movie immensely and liked not just the ridiculously loveable Graeme and Clive but also Ruth — yes please, more Wiig. And even Seth Rogen, doing very Seth Rogen goofy-pothead shtick, worked for me here. This is the rare high-concept comedy that improves as its story unfolds and had me so delighted at the end part of me actually wanted to see it again.

And for someone in the middle of a six-hour movie marathon, that’s saying something.

Pegg and Frost have a special chemistry, one that is different than the dude-buddies of Judd Apatow movies or the guys in extreme-wackiness movies like The Hangover but just as much fun — more, really. The characters here really care about each other and, while they can recognize the somewhat childish nature of their obsessions, they also have a certain maturity. It’s refreshing to see Frost and Pegg together here, giving a new flavor of goofiness, and it reminded me of why I liked them so much in Shaun of the Dead and Hot Fuzz.

Paul is a buttercream-frosted cupcake to the lovers of light sabers and Cigarette Smoking Man.

B+Rated R for language including sexual reference and some drug use. Directed by Greg Mottola and written by Nick Frost and Simon Pegg, Paul is an hour and 44 minutes long and distributed in wide release by Universal Pictures.